Insect
Evolution

Evolution
of Insects

Just
as the Paleozoic is
often called the age of the trilobite, modern time is often called the age of
the insect. The insect fossil record extends back some 400 million years to the
lower Devonian. The
Pterygotes
(winged forms) underwent a major radiation in the Carboniferous. The Endopterygota*
underwent a another major radiation in the Permian. Survivors of the mass extinction
at the PT boundary evolved in the Triassic to what are essentially the modern
Insecta Orders that persist to modern times. Most modern insect families appeared
in the Jurassic, and further diversity probably in genera occurred in the Cretaceous.
It is believed that by the Tertiary, there existed many of what are still modern
genera; hence, most insects in amber are, indeed, members of extant genera. What
seems most fascinating is that insects diversified in a brief 100 million years
(give or take) into the modern forms that exist with minor change in modern times.

Insect
evolution is characterized by rapid adaptation with selective pressures exerted
by environment. Rapid adaptation is furthered by their high fecundity. It appears
the rapid radiation's, and to this day the appearance of new species result in
insects filling all available environmental niches. Insect evolution is clodely
related to the evolution of flowering plants. Insect adaptations include feeding
on flowers and related structures
with some 20% of extant insects depend on flowers, nectar or pollen for their
food source. This symbiotic relationship is even more paramount in evolution considering
that about 2/3 of flowering plants are insect pollinated. Insects are also vectors
of many pathogens that may even have been responsible for the decimation or extinction
of some mammalian species.

Compared
to other organisms, insects have not left a particularly robust fossil record.
Other than in amber, most insects are terrestrial and only preserved under very
special conditions such as at the edge of freshwater lakes. Yet in amber, age
is limited since large resin production by trees developed later than the ancient
insects. Interestingly, while some 1/3 of non-insect species are extinct fossils,
it is believed that only 1/100th insects are extinct fossils.

*Endopterygota
is comprised by the orders of insects which are holo-metabolous, have larval
(vs.
nymphal) stages, and are separated from the Exopterygota in that they have an
internal vs. external wing development. Examples from this group are those
from Orders Coleoptera (beetles),
Hemiptera (true bugs) and Thysanoptera (thrips).